Copper immersion coating on aluminum

ABSTRACT

THE INVENTION RELATES TO A PROCESS FOR THE DEPOSITION OF A METAL ON ALUMINUM AND ALUMINUM ALLOYS BY ELECTROCHEMICAL DISPLACEMENT WITHOUT THE USE OF AN EXTERNAL CURRENT, A METHOD COMMONLY TERMED IMMERSION COATING. IT PARTICULARLY RELATES TO THE DEPOSITION OF COPPER AND TO NOVEL COMPOSITIONS AND BATHS FOR THIS PURPOSE.

United States Patent Oifi e 3,672,976 Patented June 27, 1972 3,672,976 COPPER IMMERSION COATING ON ALUMINUM Rudolf Paul Kallenbach CH8600 Duebendorf, 2 Postfach 15, Switzerland No Drawing. Filed Sept. 4, 1970, Ser. No. 69,964 Claims priority, application Germany, Sept. 17, 1969, P 19 47 003.2 Int. Cl. C23c 3/00 US. Cl. 117-130 R 8 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a process for the deposition of a metal on aluminum and aluminum alloys by electrochemical displacement without the use of an external current, a method commonly termed immersion coating. It particularly relates to the deposition of copper and to novel compositions and baths for this purpose.

BACKGROUND OF THE INVENTION Deposits of copper on material and articles manufactured of aluminum and aluminum alloys are of interest with respect to various technical applications and commercial uses. They may be applied moreover, for example, as intermediate coatings for the electrochemical deposition of other metals on aluminum and aluminum alloys. Depositing copper on aluminum and aluminum alloys by chemical or electrochemical reactions causes considerable difficulties due to the electronegative behavior of aluminum. By this it is meant the natural oxide film on aluminum and aluminum alloys impairs the adhesion of metal deposits; and, the strong reactivity of aluminum and aluminum alloys to copper in aqueous solution tends to furnish loose and spongy deposits. Methods and processes of copper coating on aluminum and aluminum alloys therefore are complicated and in particular require multistage procedures and carefully chosen operational conditions. Before coating, the protective oxide film On aluminum and aluminum alloys has to be modified or removed and the chemical activity of aluminum has to be reduced to a certain degree by appropriate means.

The state of the art is indicated by US. Pats. 2,662,831 Culverhouse, Dec. 15, 1953, and 2,872,346 Miller, Feb. 3, 1959; and other references are:

0. P. Kraemer, Metallfaerbung 3d ed. 1956, p. 51; M. Straschill, Beiztechnik 1957, p. 128;

Metal Finishing Guide Book 28th ed. 1960; and,

S. L. Matlow, J. Electrochem. Soc. 108 (1961), p. 709.

The production of a continuous deposit of copper on aluminum and aluminum alloys by electrochemical displacement from solution without use of an external current is already known.

According to one proposal in the book of O. P. Kraemer, titled Metallfaerbung (metal colouring) 3rd ed. 1956, p. 51, a stepwise treatment of the material is carried out, first in a hydrochloric copper sulfate solution and, second, in a neutral copper sulfate-chloride solution; and, according to another proposal an immersion treatment in a boiling acid copper sulfate solution is recommended. As per M. Straschill in Beiztechnik (pickling technique) 1957, p. 128, a continuous copper coating is produced using a minimum concentration of dissolved copper in sulfuric acid solution, eventually with additions of potassium-sodium tartrate. As proposed in Metal Finishing Guidebook, 28th ed. 1960, copper is deposited on aluminum and aluminum alloys in ammoniacal cyanic copper sulfate solution. As disclosed in US. Pat. 2,662,831, aluminum and aluminum alloys are coated stepwise, first with zinc, which is then replaced by copper in a bath containing complex copper fluoborate. Furthermore, from US. Pat. 2,872,346 is known the deposition of copper on aluminum and aluminum alloys by using a processing solution containing alkaline, water-soluble organic amines, S. L. Matlow proposes in Journal of the Electrochemical Society 108 (1961), p. 709 the use of ammoniacal copper chloride solution for a copper deposit on aluminum and aluminum alloys. Out of the many attempts made to produce a uniform continuous and adherent copper deposit on aluminum and aluminum alloys, none has been known to be truly satisfactory and commercially acceptable when carried into practice on a commercial scale.

It has now been discovered that, pursuant to the inventive disclosure herein, the difficulties enumerated above may be overcome and that aluminum and aluminum alloys parts may be satisfactorily coated with a continuous, adherent deposit of copper.

One object of the invention is a process and a composition to provide a copper deposit on aluminum and aluminum alloys by electrochemical displacement of dissolved bivalent copper by predominant aluminum without external current, using a slightly acid bath containing the copper partly or completely in complex binding to the oxalic group.

Another object is to provide a bath containing potassium and/ or ammonium halides in which a pH-value of about 2 to 6, preferably of about 3 to 4 is maintained and the working temperature is in the range of about 20 to C., preferably at room temperature.

An additional object is to provide in the bath a concentration of dissolved bivalent copper in the range of maximum solubility, eventually admitting the presence of copper salt as a solid phase.

Other objects of the invention will be apparent and will appear from the following description.

SUMMARY OF THE INVENTION Briefly stated, the present invention comprises a process for the production of a continuous copper deposit on aluminum and aluminum alloys by electrochemical displacement of dissolved bivalet copper by predominant aluminum without external current comprising the copper being bound partly or completely as cupric oxalic complex and the bath reacting slightly acid.

DESCRIPTION OF PREFERRED EMBODIMENTS For practical accomplishment of the processing according to the invention the material or the article of aluminum alloys first has to be subjected to common pnocedures of mechanical and/or chemical operations of cleaning, degreasing and pickling, especially in order to remove partly or completely the original oxide film on aluminum.

The requirements of the surface quality of the material before processing according to the invention comply with those of cleanliness of the metal surface before electrochemical metal plating. As pickling agents alkali, acid or acid salts of special heavy metals may be used. The respective choice chiefly depends on the type of material to be coated. In some cases by combined treatment in form of multiple stage pickling procedures special advantages may be obtained with respect to the quality of the following treatment of copper deposition.

Examples of convenient pickling agents are: neutral aqueous solutions containing about 50l00 g./l. cupric chloride 2 aq. and acid solutions containing in addition to cupric chloride up to 2.0 percent (vol.) of nitric acid. Also, a mixture of hydrofluoric acid and nitric acid (ratio 1:5) is recommended for special requirements. In other cases, intermediate treatment in diluted or concentrated nitric acid and/or in water with additional or exclusivepickling in a caustic solution of about 100-200 g./l. sodium hydroxide at room temperature or at raised temperature followed by water rinse may be used with advantage. The cleaned and pickled surface of the material should show a glossy or dull metallic appearance. Adherent deposits and contaminations, like residues from pickling treatment have to be removed by convenient means before processing in the copper bath.

A The preparation of a slightly acid solution containing a cupric oxalic complex is already known. A respective solution, for example, is set up by preserving a certain concentration of cupric halide, alkali and oxalic acid. The composition of the bath according to the invention may be changed within wide limits. The existence of water-soluble compounds of copper with the oxalic group as a cupric oxalic complex is significant to the formation of a continuous, adherent copper deposit on material of aluminum or aluminum alloys. Therefore, an oxalic compound should be present in solution with a quantity suflicient for complex binding of a major portion of the dissolved copper to the oxalic group. Some amount of alkali is essential for the adjustment of a certain acid pH-range of the bath and for the solubility of the cupric oxalic complex.

Although the composition of the bath according to the invention is not critical within wide ranges, it is adequate to select the portion of other compounds of the bath with respect to the maximum solubility of copper. Under the prevailing conditions ammonium induces a higher solubility of copper than potassium. The portion of halides activates the displacement reaction of the dissolved copper by predominant aluminum from the surface of the material and may be chosen according to the desired activity. Due to the possible variety with respect to alloying composition, cast or forging condition, heat treatment or method of manufacturing of the article the halide concentration of the bath should be adapted to suit special requirements of the material. From the group of halides the chloride has been especially effective as activating agent. In particular, the portion of chloride in solution should be less for aluminum alloys in comparison with commercial pure aluminum. Unfavorable adjustment of the chloride concentration promotes the tendency to local pitting of the metal surface in thebath. This concerns especially aluminum alloys of heterogenous composition. The effect may even be that copper is deposited as stained or discontinuous coating. In a similar way the activity of the bath should be adjusted to the type of processing material. An acid pH-range of 2-6 with preference of pH 3-4 has been found favorable to the formation of continuous adherent coating. The development of higher quantities of hydrogen gas should by avoided, and in case of occurrence the activity of the bath should be reduced. Otherwise, if the reaction of the solution with the metal surface becomes too slow the acidity of the bath may be increased.

Particularly, deposits of satisfactory quality may be obtained in the concentration limits approximately as follows: 1-8 g./l. copper, 4-16 -g./l. potassium and/or ammonium, 1-10 g./l. chloride and -80 g./l'. oxalic acid. For convenience, the bath is processed at room temperature, however, higher temperature may be chosen with the purpose to increase the rate of deposition or to reduce the period of treatment respectively.

Due to supersaturation of the solution a solid precipitated phase may be formed which contains copper preferably as cupric oxalate at the lower concentration of oxaanother way. The purpose of this provision is to maintain the portion of the dissolved copper in therange'of maximum solubility under working conditions. A decrease of the regular portion of dissolved copper in the bath during processing effects no impairment of quality of the deposit, but it may, however, reduce the rate of copper deposition. The copper deposited from solution should be replaced from time to time. This may be done by periodic addition of suitable copper salts, for example, of cupric carbonate and cupric oxalate as solid compound or as replenishing concentrate, eventually by dissolving the salt under stirring or at elevated temperature.

After extended processing enrichment of aluminum in the bath occurs. A relative low concentration of aluminum was not found to be harmful to the quality of the deposited copper.

The treatment period for material of aluminum and aluminum alloys in the bath may vary between a "few minutes and several hours depending 'on the type of material processed and as well on the desired thickness'of deposit or on the kind of commercial use of the coated material.

For various special application, especially for the purpose of passivation or of closing the pores in the copper deposit a chemical after-treatment has been found of practical value. Respective convenient procedures consist of a few minutes treatment in hot water or in diluted material chromate or dichromate solution with following rinse in water and drying. For deposits of appropriate thickness a mechanical polishing operation of the copper coated material to close the pores may be applied with success.

As illustrative of the different compositions of the bath and the variation of processing in accordance with the invention the following examples are given. Occasionally, during the treatment, a solid precipitate was built up in the bath. Under the quoted conditions of operation continuous and adherent deposits of a smoth appearance were produced on the processed material and articles.

EXAMPLE 1 Metal: Cast alloy GAlSi Pretreatment: Nitric acid-hydrofluoric acid (ratio 5:1)

30 sec. Bath:

Cupric chloride.2 aq.-6.7 g./l. Oxalic acid.2 aq.5.7 g./l. Potassium oxalate.aq.43.0 g./1. pH 4.0 Processing conditions: Room temperature, 2 hours.

After water rinsing and drying the copper coated article was electroplated in a commercial nickel bath, mechanically polished and then furnished with a chrome strike. The finish article was found of commercial quality, containing only few pores in the coating.

EXAMPLE 2 Metal: Cast alloy GAlSi 12 (alloy No. 231) Pretreatment: 200 g./l. sodium hydroxide-45 sec. Bath:

Cupric chloride.2aq.6.7 g./l.

Oxalic acid.2 aq.6.0 g./l.

Ammonium oxalate.aq.25 g./l.

pH 3.6 Processing conditions: Room temperature, 20 minutes After treatment: 10 g./l. potassium dichromate, room temperature, 10 minutes NOTE: After processing material over a longer period a concentration of 0.93 g./l. copper in the bath was analyzed. The bath was replenished with a compound of solid cupric carbonate and oxalic acid (ratio 1:5) to give a copper concentration of 1.8 g./l. copper in the bath. The throughput of material was continued until a copper concentration of 0.23 g./l. was found in the control test.

After the run a concentration of 1.1 g./l. of dissolved aluminum in the bath was tested.

EXAMPLE 3 Metal: Cast aluminm (commercial pure quality) Pretreatment:

(a) 80 g./l. cupric chloride 2 aq.i16 percent (vol.)

cone. nitric acid, 60 sec.

(b) 200 g./l. sodium hydroxide, 20 sec. Bath:

Cupric bromide-8.8 g./l.

Oxalic acid.2 aq.6.7 g./l.

Ammonium oxalate.aq.25 g. /l.

pH 3.5 Processing conditions: Room temperature, 1 hour EXAMPLE 4 Metal: Aluminum thin sheet (commercial grade soft material) Pretreatment: 100 g./l. sodium hydroxide, 20 sec.

Cupric chloride.2 aq.-6.7 g./l. Oxalic acid.2 aq.-5.7 g./l. Ammonium oxalate.aq.-25 g./l. Cupric sulfate.5 aq.-10 g./ 1. pH 3.4 Processing conditions:

Room temperature-2 hours Likewise 80 C.-20 minutes EXAMPLE 5 The advantages of the invention should be fairly apparent from the above disclosure, however, particular advantages are now described.

Copper deposits produced according to the invention are distinguished by continuity, adhesiveness and other favorable properties. The appearance of the copper coated article should be slightly dull to glossy. In a bath containing potassium, generally a deposit with a brighter glance is formed than in a bath containing preferably ammonium.

The thickness of the coating may be regulated within wide limits by adequate choice of the treatment period or of the temperature of the bath. Thin copper deposits with a thickness up to about 0.05 mm. usually exhibit satisfactory adhesion on material of aluminum and aluminum alloys. These coatings are especially suitable for reinforcement by additional deposits of copper or other metals, aplying common electrochemical plating methods.

Qualitative adhesion tests like mechanical scraping resulted in a good adhesiveness between deposit and base metal. During extrusion testing thin copper coatings on deformable sheet material adhered firmly until fissures in the base metal were arising. Metallographic testing of copper deposits on material of aluminum and aluminum alloys demonstrated the excellent throwing power obtained by processing according to the invention.

The coatings adapt well to unevenness of the surface of the base metal and are locked firmly in pits and cavities.

The coating is also formed uniformly and continuously on complicated shapes and may be deposited on areas,

for example holes, nearly inaccessible to external current using common plating methods. If desired, thicker coatings may be produced on massive articles of aluminum and aluminum alloys. However, it should be considered that copper deposits produced according to the invention are porous by their origin as follows from the principle of electrochemical displacement. The referred disadvantage may be compensated in the case of a following electrochemical plating, for example, by avoiding bath types which may attack the base metal or by inserting the copper coated article in the plating solution at switched-in external current.

According to the process described other metals with a less noble electrochemical potential than copper, for example iron, may be coated with a continuous copper deposit. Likewise, electroplating of aluminum and aluminum alloys and of other metals with a copper deposit is applicable in a solution according to the invention by inserting the material of aluminum and aluminum alloys in the electrolytical circuit as cathode.

While it will be apparent that the preferred embodiments of the invention disclosed are well calculated to fulfill the objects above stated, it will be appreciated that the invention is susceptible to modifictaion, variation and change without departing from the proper scope or fair meaning of the subjoined claims.

What is claimed is:

1. A process for the production of a continuous copper deposit on aluminum and aluminum alloys in an aqueous copper coating both by electrochemical dislacement of dissolved bivalent copper by predominant aluminum without external current comprising the copper being bound at least partly as cupric oxalic complex and the bath reacting slightly acid, said process being carried out Without the need of a reducing agent.

2. A process as set forth in claim 1, wherein the bath contains potassium and/or ammonium halides and a pH value of about 2 to about 6 is maintained.

3. A process as set forth in claim 2, wherein the temperature of the bath is maintained in the range of about 20 C. to about C.

4. Aprocess as set forth in claim 1, wherein the concentration of the dissolved bivalent copper in the bath is maintained in the range of maximum solubility.

5. A process as set forth in claim 1, wherein the bath contains potassium and/ or ammonium halides and a pH value of about 3 to about 4 is maintained.

6. A process as set forth in claim 1, wherein the temperature of the bath is maintained in the range of about 20 C. to about 90 C.

7. A process as set forth in claim 6, wherein the concentration of the dissolved bivalent copper in the bath is maintained in the range of maximum solubility.

8. A process as set forth in claim 7, wherein the bath contains potassium and/or ammonium halides and a pH value of about 2 to about 6 is maintained.

References Cited UNITED STATES PATENTS 3,147,154 9/1964 Cole et al. 117-130 E X 3,141,780 7/1964 Simon et al. 106-1 3,403,035 9/1968 Schneble et al. 117-130 E X 3,178,311 4/1965 Cann 106-1 X 3,531,301 9/1970 Watson 117-130 E X RALPH S. KENDALL, Primary Examiner C. WESTON, Assistant Examiner US. Cl. X.R. 117-160 R 

